High power radiation emitting devices, such as semiconductor lasers, typically comprise a body of semiconductor material having opposed end faces in which an active layer is positioned between two cladding regions of opposite conductivity. Gain which is necessary for these high power devices results from a population inversion which occurs when applied current is increased. The end faces of the body form a resonant cavity such that radiation generated in the active layer is partially reflected back into the semiconductor body by an end face toward the opposing end face. When the current is sufficiently increased above some threshold value the increase in gain causes lasing action to occur. Lasers emit a narrow band of highly coherent radiation having a coherence length of approximately 2 centimeters(cm). Coherent radiation, or radiation having a high spectral modulation, is undesirable in some applications, such as fiber optic gyroscopes, which require high power devices which emit radiation having low coherence. Other devices such as light emitting diodes (LED's) emit a broad band of radiation but operate at low power, insufficient for high power applications.
Super-luminescent diodes (SLDs) provide a high power output of broad band low coherent radiation, that being radiation having a coherence length of less than about 200 micrometers (.mu.m) and typically about 50 .mu.m. An SLD typically has a structure similar to that of a laser, with lasing being prevented by antireflection coatings formed on the end faces. These coatings must reduce the reflectivity of the end faces to about 10.sup.-5 or less to prevent lasing in a high power SLD and further, this reflectivity must be reduced to about 10.sup.-6 or less to achieve low spectral modulation. Spectral modulation is the percentage ratio of the difference between the maximum and minimum power output divided by the sum of the maximum and minimum power output and low spectral modulation is 5% or less modulation. Unfortunately, low reflectivity of about 10.sup.-6 at the end faces is difficult to obtain consistently for a given output wavelength and even a slight temperature change which alters the output wavelength will change the reflectivity, thus making the manufacture of a low spectral modulation SLD extremely difficult.
Other SLD structures utilize a stripe interrupt geometry in which a metallized stripe is formed over a portion of an active region. This stripe extends from one end face towards but not up to the opposing end face. During device operation of these SLDs a reflecting interface is formed under the end of the metallized stripe which does not extend to the opposing end face due to differences in propagation characteristics in the active region, where current is not supplied by the metallized stripe. This interface results in high spectral modulation even at moderate power levels.
Due to the aforementioned problems, an SLD has been limited to a maximum output power of about 7 mw continuous wave (cw) and has had high spectral modulation, typically 50% at maximum power and 20% at half power. Thus, it would be desirable to have an alternative construction for SLDs and method for making same.